The present invention relates to a method and apparatus for cooling or utilizing hot gas in a reactor in which the lower section of the reactor is provided with a hot gas inlet and a chamber encompassing a fluidized bed, the middle section is provided with a riser, and the upper section with a gas outlet, and the reactor has heat transfer surfaces for recovering heat from solid particles. The invention especially relates to a method, in which hot gas is introduced through the inlet into the lower section of the reactor, and solid particles from the bubbling fluidized bed are fed to the inlet gas for cooling thereof, solid particles are separated from the cooled gas and returned to the fluidized bed, heat is recovered from the separated solid particles, and the cooled gas is discharged through the gas outlet.
Fluid bed reactors are well suited to cooling of hot gases containing molten and/or vaporized components and/or tar-like particles. Gas coolers are suited to, e.g., cooling of exhaust gases from industrial plants and dry purification of gases from partial oxidation of biomass, peat or coal containing dust and tar and other condensing components. The hot gases introduced into the reactor are efficiently cooled by mixing solid particles therewith, such solid particles having been cooled earlier in the reactor.
Finnish patent 64997 teaches cooling of hot gases in circulating fluidized bed reactors. Here hot gases are fed as fluidizing gas into the mixing chamber of the reactor, where the gases cool efficiently as they come into contact with a large volume of solid particles, i.e., bed material. Solid particles are carried by the gas flow through the riser into the upper section of the reactor, where they are separated and then returned to the fluidized bed in the mixing chamber. In the riser, the gas flow conveying solid particles may be cooled by heat transfer surfaces.
A drawback of the method described above is, however, that the hot gases to be cooled have to fluidize a large volume of solid particles, resulting in a high power requirement. On the other hand, a sudden interruption in the power supply may result in the entire bed flowing through the inlet and then out of the reactor.
Finnish patent application 913416 also teaches cooling of hot process gas during stationary fluidization, i.e., a bubbling fluidized bed. here the hot gas flowing into the reactor is supplied with solid particles as an overflow from the bubbling fluidized bed. The gas and the solid particles entrained therewith flow into a dust collector disposed above the bubbling fluidized bed, from which solid particles then drop back onto the surface of the bubbling fluidized bed as the flow rate of the gas quickly decreases. The bubbling fluidized bed and the gas riser, which is disposed above the dust collector, are provided with heat transfer surfaces.
In the arrangement described above, the particles falling onto the surface of the bubbling fluidized bed are carried along the surface back to the overflow point, where they are immediately taken to recirculation, ending up in the dust collector. Thus, a separate "surface circulation" of hot particles develops above the fluidized bed. These particles do not cool efficiently in the fluidized bed because the particles which are deeper down in the fluidized bed, near the heat transfer surfaces, cannot mix efficiently with the particles present in the "surface circulation".
In the method described above, the riser is considered a natural place for the heat transfer surfaces because the solids and gas flows are swift in the riser. The gas stream, however, causes wear of the heat transfer surfaces in the riser. Wear is partly attributable to the composition of the gas as well as to the dust contained in it, and partly to the high flow rate of the gas.
In some cases, the hot gas flowing to the separator may cause fouling and clogging of the heat transfer surfaces when the gas enters the heat transfer surfaces at too high a temperature. If the hot gas does not cool until it touches the heat transfer surfaces, the impurities will correspondingly condense on or adhere to these surfaces, and not on the circulating mass particles as intended.
Chlorine-containing gases, in particular, cause corrosion at high temperatures and, therefore, superheating of steam to high temperatures is not usually possible in the heat transfer surfaces of the riser. SO.sub.3 may cause problems with the heat transfer surfaces at low temperatures.
According to the present invention an improved method and apparatus, when compared with the above-described methods and apparatus, for cooling or utilizing hot gases in the hot gas treatment of solid material are provided. The method and apparatus of the invention are provided to minimize power consumption and wear of the heat transfer surfaces.
The method and apparatus provide means by which the heat energy released by the hot gas when it cools may be utilized as efficiently as possible, e.g., for generation of superheated steam, without a substantial risk of corrosion.
The invention provides a method and apparatus for substantially decreasing the corrosion of the heat transfer surfaces caused by components, such as chlorine, contained in the gas, and thus utilizes more efficiently the heat energy released by the hot gas when it cools. [For example, for the generation of superheated steam.] The invention also provides fast and effective cooling of the gases.
According to one aspect of the present invention, there is provided a method of cooling hot gas (e.g. typically at a temperature greater than 400 degrees C to about 200-400 degrees C, or below) from about 1000-1300 degrees C in a reactor having lower and upper sections, and having an inlet duct and a mixing chamber centrally located in the lower section and a fluidized bed in the lower section of the reactor radially outward from the inlet duct and mixing chamber, a riser from the mixing chamber, and a gas outlet in an upper section of the reactor and a particle separator in communication with the upper section of the reactor. The method comprising the following steps: (a) Introducing hot gas into the mixing chamber through the inlet duct, the gas flowing upwardly through the mixing chamber where it comes into contact with and entrains cooling particles, and then flows into the riser. (b) Separating particles from gas in the separator and returning the separated particles toward the fluidized bed. (c) Introducing some returning particles from step (b) directly into the mixing chamber and others into the fluidized bed. (d) Cooling the particles in, or prior to return to, or both in and prior to return to, the fluidized bed; and (e) introducing some cooled particles from the fluidized bed into the mixing chamber so as to contact and mix with the hot gas introduced in step (a) and effect cooling thereof. The method typically also comprises the further step of cooling the particles during the practice of step (b) as the particles are being returned so that the particles introduced by step (c) directly into the mixing chamber have been cooled. Cooling is practiced to cool the tar and like components of the hot gas so that they are below the temperature at which they are tacky and impede flow (by sticking onto surfaces of the reactor components, etc.), e.g. below 400 degrees C.
According to another aspect of the invention there is provided an apparatus for cooling hot gas is provided comprising the following components: A reactor having a lower section and an upper section. A gas outlet in the upper section. A gas inlet duct located centrally in the lower section of the reactor. A mixing chamber, in which hot gas and cooling particles are mixed, in the lower section of the reactor above the inlet duct, the mixing chamber substantially concentric with the gas inlet duct. A riser connecting the mixing chamber and the upper section of the reactor. A particle separator in operative communication with the upper section of the reactor for separating particles from gas. A return duct for returning particles from the particle separator toward the lower section of the reactor. A fluidized bed of particles connected to the return duct. Means for introducing particles from the return duct directly into the mixing chamber, and into the fluidized bed. Means for cooling separated particles in, or prior to return to, or both in and prior to return to, the fluidized bed and means for introducing some cooled particles from the fluidized bed into the mixing chamber so as to contact and mix with the hot gas introduced into the inlet duct and effect cooling thereof. The introducing means for introducing particles from the fluidized bed into the mixing chamber may comprise a loop seal, J seal, gill seal, valve, baffle assembly, conduit, diverter, or the like which prevent reverse flow to the fluidized bed. The means for introducing particles from the return duct directly into the mixing chamber and into the fluidized bed may comprise a baffle assembly, differently directed conduits, diverter, or the like.
According to a preferred embodiment of the invention, solid particles are preferably conveyed from the fluidized bed through the solids openings provided in the lower section thereof into the hot gas flow in the mixing chamber, in the wall between the mixing chamber and the fluidized bed. Due to a higher static pressure in the fluidized bed, solid material may be caused to flow automatically through the openings into the hot mixing chamber, but the solids flow may also be regulated by feeding fluidizing gas into the openings, which prevents flowing of the gas from the mixing chamber to the fluidized bed against the flow direction of the solids. In this way, it is possible to regulate the flow of solid particles.
In the reactor according to the invention, hot gas is cooled to a substantially lower temperature immediately at the mixing chamber by mixing cooled solid particles with the gas, so that the gas cools and the solid particles are correspondingly heated. Besides cooling of gases, the invention may be employed in processes where solid material is heated or otherwise treated with hot gases, such as, e.g., heating of lime with hot gases.
In a reactor according to a preferred embodiment of the invention, gas may also be cooled by constructing the mixing chamber and the riser of cooled surfaces. Solid particles are separated from the gas in a particle separator. The solid particles are conveyed as a dense suspension, almost as a plug flow if desired, via the return duct back to the lower section of the reactor. In the return duct there preferably is disposed heat recovery surfaces for recovering the heat energy released by heated solid particles, and they may be connected to other heat exchangers, a turbine, or the like. According to the invention, solid particles are returned to the mixing chamber in the lower section of the reactor, into the gas to be cooled. The return duct is preferably provided with means for leading the returning solid particles directly to the mixing chamber, and to the fluidized bed.
Proper control of the circulating solids flow improves the controllability and increases the reaction velocity of the process. Furthermore, the circulating fluidized bed maintains the reactor surfaces clean, ensuring that clogging does not occur, so that cooling of the gas is always certain when the cooling of the solids functions reliably.
The return duct is a favorable location for heat transfer surfaces because the particle density is relatively high there, which is beneficial for heat transfer. Hot gas containing molten or condensing components, which might clog the heat transfer surfaces, also does not significantly flow into the return duct.
Heat transfer surfaces may also be disposed in the fluidized bed itself, where the flow is slow and thereby favorable to the durability of the heat transfer surfaces. Also such gas that provides favorable conditions, e.g., inert gas, air or other gas containing non-corroding substance, may be supplied to the fluidized bed as a fluidizing gas. Also heat exchange is efficient due to a high particle density.
The method and apparatus according to the invention provide efficient mixing of solids and hot gas and, consequently, efficient heat exchange from the gases to the solid material. Furthermore, the method and apparatus according to the invention provide a simple arrangement for minimizing wear of the heat transfer surfaces in the gas cooler. At the same time, power consumption is capable of being lowered in comparison with the prior art. Furthermore, in the arrangement according to the invention, the heat energy released by the gases is well utilized, e.g., by generating superheated steam.